1. Field of the Invention
The present invention relates to the use of a high energy electron beam to chemically transform or destroy certain types of hazardous waste, and more particularly, to an electron gun that provides a broad diverging electron beam that can be efficiently transmitted into a detoxification vessel.
2. Description of Related Art
Volatile organic compounds (VOCs) exist in the form of vapors or gasses that are emitted or vaporized from hazardous or toxic waste materials. Since these VOCs pose a significant health risk to individuals and to the environment, it is necessary to contain, extract and collect the hazardous materials so as to prevent spreading of the VOCs into the air and/or ground water. Once contained, the VOCs can be remediated by converting them into less hazardous materials that can be disposed of with substantially reduced risk.
One such remediation technique involves the injection of a high energy electron beam into a detoxification vessel containing the VOCs. Interaction between the electrons of the beam and the VOCs causes chemical transformation of the VOCs in three significant aspects, including: (1) direct de-chlorination resulting in inorganic chloride ions and reactive organic intermediates which are further degraded into non-reactive compounds; (2) production of organic and inorganic free radicals and ions which are reactive and whose reactions result in destruction of the target hazardous materials; and (3) formation of aqueous electrons (in the presence of water vapor) capable of reducing chemical bonds. An example of a toxic remediation device comprising an electron beam coupled into a detoxification vessel is disclosed in U.S. Pat. No. 5,319,211.
In order to achieve a sufficient level of remediation within the detoxification vessel, it is desirable to provide a broad electron beam having relatively high energy (in excess of 160 kilovolts DC). The source of the electron beam typically comprises a tungsten filament and a positively biased control grid. The tungsten filament emits a broad, unfocused stream of electrons in response to a positive voltage applied to the control grid. The control grid also focuses the beam into isolated beamlets having non-uniform current density. A set of magnetic coils can be used to move the beam over the surface of a target grid that separates the electron beam source from the detoxification vessel in order to keep the target grid from overheating at any one spot.
A significant drawback of this type of electron beam source is that the filament is operated at a rather high temperature (in excess of 1,700.degree. C.) and filament power level. The control grid must also dissipate a relatively high intercept current which is manifested in the form of excess heat. Unless the high heat level generated by the filament and control grid can be efficiently removed from the device, the operating efficiency, life and reliability of the device is degraded. In some cases, the heat from the filament has been known to cause damage to the high voltage connection to the device. Moreover, the inherent inefficiency associated with using a tungsten filament as an electron source renders it entirely impractical to operate more than one such beam source in parallel due to the extremely high filament power requirement.
An alternative approach is to utilize a conventional electron gun of the type commonly used in linear beam devices, such as klystrons and traveling wave tubes, as the electron beam source. Such electron guns include a thermionic dispenser cathode, and are capable of efficiently producing a high energy electron beam. The shape of this electron beam, however, is not optimum for use in toxic remediation. Instead of producing a broad, diverging electron beam, prior art electron guns produce a converging beam that tends to concentrate all the energy of the beam into a relatively small diameter region of the detoxification vessel. The concentrated beam produces localized hot spots by interception with the target grid and the surfaces of the detoxification vessel. As a result, the interaction between the electrons and the VOCs is less than optimum and a significant portion of the energy of the beam is dissipated in the form of thermal loss. Further, the electron gun must operate at a relatively low beam current in order to keep the current density (w/cm.sup.2) measured at the target grid below an acceptable level.
Thus, a critical need exists for an efficient electron beam source to provide a diverging beam that can be introduced into a detoxification vessel with uniform current density over a wide surface. Such an electron beam source would enable higher transmitted energy and better interaction of the electron beam with the VOCs of the detoxification vessel.